Substrate cleaning method and substrate cleaning apparatus

ABSTRACT

In a cleaning treatment of a substrate using an aqueous solution of ammonium fluoride or a mixture of an aqueous solution of ammonium fluoride and hydrofluoric acid as a cleaning liquid, the cleaning liquid is replenished by at least one liquid selected from the group consisting of water, ammonia, aqueous ammonia, and an aqueous solution of ammonium fluoride with the lapse of time during the use of the cleaning liquid, in which the required amount of the liquid to be added according to the time lapse is calculated based on the measurement date and controlled, or the component concentration of the cleaning liquid is detected and the liquid is added according to the obtained result, so that not only can the substrate be cleaned uniformly and stably, but also a resource saving and a reduction in waste can be achieved.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] The present document is based on Japanese Priority Documents JP2000-214974, filed in the Japanese Patent Office on Jul. 14, 2000 and JP2000-240134 filed on Aug. 8, 2000 the entire contents of which beingincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a substrate cleaning method anda substrate cleaning apparatus for cleaning a substrate, which use, as acleaning liquid, an aqueous solution of ammonium fluoride or a mixtureof an aqueous solution of ammonium fluoride and hydrofluoric acid. Moreparticularly, the present invention is concerned with a novel substratecleaning method and a novel substrate cleaning apparatus, which aredeveloped for achieving both uniform and stable cleaning treatment andreduction in the amount of the cleaning liquid used.

[0004] 2. Description of the Related Art

[0005] An aqueous solution of ammonium fluoride (including arbitrarymixtures of hydrofluoric acid and ammonia, and ones containing asurfactant) is widely used in the form of a mixture thereof withhydrofluoric acid and the like for cleaning or etching a semiconductor(mainly a silicon oxide film) substrate or a glass substrate in theproduction process for semiconductor or liquid crystal display (LCD).

[0006] In addition to the above example of substrate cleaning, inUnexamined Japanese Patent Application Laid-Open Specification No.2000-150447, a mixture of ammonia and hydrogen peroxide is used forcleaning silicon wafers. This prior art document discloses that theammonia concentration and hydrogen peroxide concentration of the mixtureare individually monitored during the cleaning, and the change with timeof the chemical liquid concentration is accurately estimated based onthe results of measurements to calculate and adjust the amount of thechemical liquid to be added, so that the chemical liquid concentrationis kept constant.

[0007] In these fields, for achieving reduction in weight and downsizingof products and the lowering of consumed electric power, development ofa microfabrication technology for a higher integration degree isdesired.

[0008] In any case of cleaning substrates using, as a cleaning liquid,for example, an aqueous solution of ammonium fluoride, a mixture of anaqueous solution of ammonium fluoride and hydrofluoric acid as mentionedabove, or other chemical liquids, a higher accuracy treatment isdesired.

[0009] By the way, in the above-mentioned cleaning or etching, thechemical component content, such as ammonium fluoride (NH₄F), andhydrogen fluoride (HF), and moisture content of the cleaning liquidchange (evaporate) with the lapse of time. Therefore, such cleaning oretching has a problem in that the etching rate for a silicon oxide filmor a glass substrate changes (increases).

[0010] This is caused by the gradual increase in the hydrofluoric acidcomponent concentration of the cleaning liquid with the lapse of timeduring the use of the cleaning liquid, and the problem of the change inthe concentrations of the components in the cleaning liquid has beenconventionally solved by frequently replacing the cleaning liquid.

[0011] However, in this method, a great amount of the cleaning liquid isrequired. Especially a cleaning liquid containing ammonium fluoride isused in an ammonium fluoride concentration as high as several tens %(for example, about 40% by weight), and hence, the amount of thechemical consumed per liquid replacement in this cleaning liquid islarge, as compared to those in other cleaning liquids generally used ina chemical concentration of about several %. Accordingly, the amounts ofthe ammonium fluoride and hydrofluoric acid used (cost for chemicals)are to increase.

[0012] Further, after the cleaning liquid is used, a waste watertreatment shown in FIG. 1 is needed, and, in this treatment, a largeamount of resource (waste water treatment agent) is consumed, resultingin a large amount of waste products (waste water and sludge)(see FIG.2).

[0013]FIG. 1 is an explanatory view illustrating a process for treatingthe effluent of a cleaning liquid. Effluent produced in a cleaning bath11 is transferred to a pH-adjusting bath 12, and neutralized in thisbath by, for example, a 20% aqueous solution of calcium hydroxide. Next,the effluent is transferred to a coagulo-sedimentation bath 13 andsubjected to coagulation and sedimentation by a chemical, such asaluminum sulfate, and then, in a flocculation bath 14, is flocculated bya polymeric flocculant, such as a polyacrylamide flocculant, andsubjected to sedimentation, followed by discharge in the form of sewageand sludge.

[0014] In the above treatment process, as shown in FIG. 2, for treating1 kg of a 40% aqueous solution of ammonium fluoride, 2.0 kg of 20%calcium hydroxide, 0.3 kg of 8% aluminum sulfate, and 1.6 kg of apolymeric flocculant are required, so that 2.6 kg of sludge having asolids content of 70% and 2.3 kg of sewage waste water are produced.

[0015] In recent years, concern about the environmental problems isgrowing worldwide, and a demand for removing the environmental burdencaused by the production of substrates for semiconductor and LCD isbeing increasing. Therefore, with respect to the cleaning liquid for thesubstrates, not only for reducing the cost, but also for achievingpreservation of the environment, i.e., a saving of resource, a reductionin waste, and environmental cleaning, a solution of the problem aboutthe reduction in the amount of the cleaning liquid used is sociallydesired strongly.

SUMMARY OF THE INVENTION

[0016] According to the present invention, there is provided a substratecleaning method and a substrate cleaning apparatus which enable aresource saving and a reduction in waste to be achieved.

[0017] The present invention also provides a substrate cleaning methodwhich is developed for achieving both the uniform and stable cleaningtreatment and the reduction in the amount of the cleaning liquid used.

[0018] Furthermore, the present invention provides a substrate cleaningapparatus which is developed for achieving both uniform and stablecleaning treatment and reduction in the amount of the cleaning liquidused.

[0019] Still furthermore, a substrate cleaning method and a substratecleaning apparatus, which use an aqueous solution of ammonium fluorideor a mixture of an aqueous solution of ammonium fluoride andhydrofluoric acid as a cleaning liquid is also provided according to thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above and other objects, features and advantages of thepresent invention will become more apparent from the followingdescription of the presently preferred exemplary embodiments of theinvention taken in conjunction with the accompanying drawings, in which:

[0021]FIG. 1 is an explanatory view illustrating a process for treatingan effluent of a cleaning liquid, which is the related art.

[0022]FIG. 2 is a diagram showing a resource (chemicals) required for aneffluent treatment for a cleaning liquid, which is the related art.

[0023]FIG. 3 is a graph showing the relationship between time andetching rate with respect to the cleaning liquid.

[0024]FIG. 4 is a graph showing the relationship between the time andthe HF concentration of the cleaning liquid.

[0025]FIG. 5 is a diagrammatic view showing an example of a substratecleaning apparatus, which is a first embodiment of the presentinvention.

[0026]FIG. 6 is a graph showing the change in the HF concentration byaddition of water according to the present invention.

[0027]FIG. 7 is a graph showing the change in the etching rate byaddition of water according to the present invention.

[0028]FIG. 8 is a graph showing the state of controlling the etchingrate and the HF concentration by addition of water according to thepresent invention.

[0029]FIG. 9 is a diagrammatic view showing a substrate cleaningapparatus, which is a second embodiment of the present invention.

[0030]FIG. 10 is a diagrammatic view showing an example of a substratecleaning apparatus in which an aqueous ammonia storage tank is provided,which is a third embodiment of the present invention.

[0031]FIG. 11 is a diagrammatic view showing an example of a substratecleaning apparatus in which an aqueous ammonia storage tank is provided,which is a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Hereinbelow, a substrate cleaning method and a substrate cleaningapparatus according to the present invention will be described in detailwith reference to the accompanying drawings.

[0033] In the present invention, in the cleaning (or etching) treatmentof a substrate using, as a treatment liquid (or a cleaning liquid), anaqueous solution of ammonium fluoride or a mixture of an aqueoussolution of ammonium fluoride and hydrofluoric acid, the cleaning liquidis replenished by at least one liquid selected from the group consistingof water, ammonia, aqueous ammonia, and an aqueous solution of ammoniumfluoride according to the accumulative time for using the cleaningliquid and the individual component concentrations of the cleaningliquid.

[0034]FIG. 3 shows the relationship between the lapse of time and theetching rate for a thermal oxide film with respect to the mixture ofammonium fluoride and hydrofluoric acid. The cleaning liquid has acomposition such that the NH₄F (40%):HF (50%) ratio is 400:1, and thetemperature of the cleaning liquid is 25° C. and the thermal oxide filmis comprised of SiO₂.

[0035] In the graph shown in FIG. 3, the time is taken as the abscissaand represented by a unit of “days”, and the etching rate for asubstrate is taken as the ordinate and represented by a unit of theetching rate per minute, i.e., “Å/min”. The etching rate changes withtime as follows: after a lapse of 0 day, the etching rate is about 34Å/min; after a lapse of 11 days, the etching rate is 70 Å/min; and aftera lapse of 22 days, the etching rate is 95 Å/min.

[0036] As can be seen from FIG. 1, the etching rate for the thermaloxide film remarkably increases with the lapse of time.

[0037] As mentioned above, in the cleaning treatment of a substrateusing the above-mentioned cleaning liquid, the etching rate changes withtime, and there is a very good correlation (proportionality relation)between the etching rate and the time. This means that the moisturecontent and chemical component (such as ammonium fluoride (NH₄F) andhydrogen fluoride (HF)) contents of the cleaning liquid change(evaporate) with the lapse of time, and specifically means that thewater component and ammonia component of the cleaning liquid evaporateat a constant rate, so that the concentration of the hydrogen fluoride(HF) component (which directly contributes to the etching for thethermal oxide film) in the cleaning liquid increases at a constant rate.

[0038]FIG. 4 shows the change in the HF concentration of the cleaningliquid with the lapse of time.

[0039] The time is taken as the abscissa and represented by a unit of“days”, and ranges from 0 to 25 days. On the other hand, the HFconcentration of the mixture is taken as the ordinate and represented bya unit of “% by weight (wt %)”, and ranges from 0 to 0.6 wt % and isshown in a scale of 0.1 wt %.

[0040] After a lapse of 0 day, the HF concentration is 0.1 wt %; after alapse of 11 days, the HF concentration is 0.35 wt %; and after a lapseof 22 days, the HF concentration is 4.7 wt %.

[0041] From the above, it is found that the HF concentration of themixture increases at a constant rate with the lapse of time.

[0042] Thus, in the cleaning (etching) treatment of a substrate using anaqueous solution of ammonium fluoride or a mixture of an aqueoussolution of ammonium fluoride and hydrofluoric acid, the HFconcentration of the cleaning liquid is required to be kept constant. Inthis case, a most effective method is the one in which water thatreadily evaporates from the cleaning liquid is appropriately added tothe cleaning liquid.

[0043] Specifically, when water is added to the cleaning liquid with thelapse of time, a rise in the HF concentration can be suppressed, so thatthe etching rate can be kept constant. Also when an aqueous solution ofammonium fluoride is added to the cleaning liquid, the HF concentrationis lowered, so that a similar effect can be expected.

[0044] On the other hand, when ammonia is added to the cleaning liquid,HF in the cleaning liquid is neutralized by ammonia to form ammoniumfluoride, so that a rise in the HF concentration of the cleaning liquidcan be suppressed. When aqueous ammonia is added, both the dilutioneffect and the neutralization effect can be expected.

[0045] By the above-mentioned method, not only can the yield ofsubstrates for semiconductor and liquid crystal be improved, but alsothe frequency of the liquid replacement can be reduced, thus making itpossible to achieve both the saving of chemicals, such as the cleaningliquid and waste water treatment agent, and the reduction in sludge andwaste water amount.

[0046]FIG. 5 shows an example of the construction of the substratecleaning apparatus according to the present invention.

[0047] This substrate cleaning apparatus comprises a substrate treatmentbath 21 for storing therein a cleaning liquid comprising, for example, amixture of ammonium fluoride and hydrofluoric acid, and for subjecting asubstrate to cleaning treatment, and a circulation pump 22 forcirculating the cleaning liquid which is the overflow back to thesubstrate treatment bath 21. For example, a substrate, which is asubstance to be treated, is accommodated in a substrate carrier anddipped in the cleaning liquid in the substrate treatment bath 21, andthus performing a cleaning (etching) treatment.

[0048] The characteristic feature of the above apparatus resides in thatthe substrate treatment bath 21 is provided with a constant deliverypump 23 and a controller 24, by which feeding water and addition ofwater through the pump 23 is controlled.

[0049] As mentioned above, the HF component concentration of thecleaning liquid increases with the lapse of time at a constant rate, sothat the etching rate for the thermal oxide film remarkably increases.

[0050] Therefore, in this substrate cleaning apparatus, the HFconcentration is controlled by the controller 24 so as to be keptconstant by adding water to the substrate treatment bath 21 with thelapse of time by means of the constant delivery pump 23 so that theetching rate is kept substantially constant.

[0051]FIG. 6 shows the change in the HF concentration when water isadded. In this graph, the time is taken as the abscissa and representedby a unit of “hours (hrs)”, and ranges from 0 to 300 hrs. On the otherhand, the HF concentration is taken as the ordinate and represented by aunit of “wt %”, and ranges from 2.92 to 3.06 wt %. After a lapse of 0hr, the HF concentration is 2.94 wt %; and after a lapse of 220 hrs, theHF concentration is 3.04 wt %. When water is added at this point intime, the HF concentration is lowered to 2.95 wt %. After a lapse offurther about 20 hrs, the HF concentration slightly increases to 2.96 wt%.

[0052] As apparent from the above description, the HF concentrationwhich increases with the lapse of time goes back to the originalconcentration by addition of water. The control of the HF concentrationby addition of water leads to the control of the etching rate, and, asshown in FIG. 7, the etching rate is also lowered by the addition ofwater.

[0053] With respect to the amount of the water added and the timing ofthe water addition, data is obtained as shown in FIG. 3 and FIG. 4 tocalculate the amount of the water which evaporates with the lapse oftime, making it possible to introduce the optimal amount of the wateradded and the optimal conditions for the timing of the water addition.

[0054]FIG. 7 is a graph showing the state of controlling the etchingrate and HF concentration by addition of water.

[0055] The time is taken as the abscissa and represented by a unit of“hours (hrs)”, and ranges from 0 to 250 hours. On the other hand, theetching rate is taken as the ordinate and represented by a unit of“nm/min”, and ranges from 65 to 85 nm/min.

[0056] After a lapse of 25 hours, the etching rate is 74 nm/min; after alapse of 70 hours, the etching rate is 76 nm/min; and after a lapse of220 hours, the etching rate is 82 nm/min. When water is added at a pointin time after 220 hours, the etching rate is lowered to about 72 nm/min.

[0057] Thus, the etching rate and the HF concentration can be kept in aconstant range by intermittently adding water.

[0058] A graph showing the relationship between the etching rate and thehydrofluoric acid concentration when water is intermittently added isillustrated in FIG. 8.

[0059]FIG. 8 shows an example in which water is intermittently added,but the conditions for water addition may be any of continuous additionand intermittent addition. For keeping the concentration constant, thecontinuous addition is preferred.

[0060] In the above substrate cleaning apparatus, the timing of theaddition of water is controlled based on the measurement data, but, asshown in FIG. 9, a concentration measuring unit 35 can be provided tomeasure the concentration of the cleaning liquid being circulated, sothat the addition of water can be controlled based on the concentrationdata in real time.

[0061] Specifically, the concentrations of the components (e.g.,ammonia, hydrofluoric acid, and water) in the cleaning liquid areindividually measured by means of the concentration measuring unit 35,and the measurement result is transmitted to a controller 34 comprisedof a computer and a central monitoring unit to determine the additionand calculate the required amount of addition, followed by transmissionof an instruction for replenishment to a water feeding line (constantdelivery pump 33). Then, whether or not the addition is made inaccordance with the instruction is confirmed by a measurement by meansof the concentration measuring unit 35.

[0062] In the present invention, as a method for measuring theconcentrations of the components in the cleaning liquid, a measurementof an absorbance at a predetermined wavelength, an infrared absorptionspectrum, an ultraviolet absorption spectrum, an index of refraction, aspecific gravity, a transmittance, or an electric conductivity may beperformed and a measuring unit, such as a Karl Fischer moisture titratoror liquid (ion) chromatography may be used.

[0063]FIG. 10 and FIG. 11 show an example in which an aqueous ammoniastorage tank 47 or 57 for feeding aqueous ammonia is provided instead ofthe constant delivery pump 23 or 33 for feeding water in FIG. 5 and FIG.9, respectively.

[0064] By adding aqueous ammonia instead of water, the HF concentrationis effectively controlled, so that the etching rate is controlled to bekept constant.

[0065] In the feeding of ammonia, not only aqueous ammonia but alsoother appropriate aqueous solutions can be used, and further, ammoniagas can also be used. Alternatively, an aqueous solution of ammoniumfluoride can be used, and ammonium fluoride can be used in combinationwith ammonia.

[0066] As mentioned above, by adding to the cleaning liquid water,aqueous ammonia, or an aqueous solution of ammonium fluoride inconsideration of the etching rate and the individual componentconcentrations of the cleaning liquid at a certain point in time in thetreatment, the etching treatment by the cleaning liquid can be conducteduniformly and stably (namely, the etching rate for an oxide film can bekept constant), so that there is no need to frequently replace thecleaning liquid differing from the conventional cleaning. Therefore, thelife time of the cleaning liquid can be prolonged, and the reduction infrequency of the liquid replacement contributes to a reduction in thecleaning liquid consumed (resource saving) and a reduction in the wastewater treatment agent required for the waste water treatment of thecleaning liquid (resource saving). Further, the reduction in thesechemicals used contributes to a reduction in the amount of sludge andwaste water produced upon the waste water treatment (reduction in theamount of waste).

[0067] Thus, the present invention not only achieves the uniform andstable substrate treatment but also contributes to resource saving andthe reduction in the amount of waste, namely, preservation of theenvironment.

[0068] As mentioned above, by using the method and apparatus of thepresent invention, in the cleaning of a substrate using an aqueoussolution of ammonium fluoride or a mixture of an aqueous solution ofammonium fluoride and hydrofluoric acid, not only can the treatment beconducted uniformly and stably, but also the frequency of replacement ofthe cleaning liquid can be reduced, thus making it possible to achievethe saving of resource, i.e., chemicals (a cleaning liquid and a wastewater treatment agent), and considerably reduce the amount of sludge andwaste water produced upon the treatment.

[0069] Although the invention has been described in its preferred formwith a certain degree of particularity, obviously many changes andvariations are possible therein. It is therefore to be understood thatthe present invention may be practiced otherwise than as specificallydescribed herein without departing from the scope and the sprit thereof.

What is claimed is:
 1. A substrate cleaning method for cleaning asubstrate using a cleaning liquid comprising at least one selected fromthe group consisting of an aqueous solution of ammonium fluoride, and amixture of an aqueous solution of ammonium fluoride and hydrofluoricacid, comprising the steps of: dipping and cleaning the substrate insaid cleaning liquid; and adding a replenishing liquid comprising atleast one selected from the group consisting of water, ammonia, aqueousammonia, and an aqueous solution of ammonium fluoride with lapse of timeduring said cleaning liquid is used.
 2. The substrate cleaning methodaccording to claim 1, further comprising the step of measuringcharacteristics of said cleaning liquid, wherein a kind and an amount ofsaid replenishing liquid are determined in response to a result of themeasurement of said cleaning liquid.
 3. The substrate cleaning methodaccording to claim 2, wherein, in the measuring step, hydrofluoric acidconcentration is measured at a predetermined time interval and water isfed as said replenishing liquid so that a measurement value of saidconcentration falls within a predetermined range.
 4. The substratecleaning method according to claim 2, wherein said characteristics ofsaid cleaning liquid is measured by at least one measurement selectedfrom the group consisting of measurements of an absorbance at apredetermined wavelength, an infrared absorption spectrum, anultraviolet absorption spectrum, an index of refraction, a specificgravity, a transmittance, and an electric conductivity, a measurement bymeans of a moisture titrator, and a measurement by means of liquid (ion)chromatography.
 5. The substrate cleaning method according to claim 3,wherein said hydrofluoric acid concentration of said cleaning liquid ismeasured by at least one measurement selected from the group consistingof measurements of an absorbance at a predetermined wavelength, aninfrared absorption spectrum, an ultraviolet absorption spectrum, anindex of refraction, a specific gravity, a transmittance, and anelectric conductivity, a measurement by means of a moisture titrator,and a measurement by means of liquid (ion) chromatography.
 6. Asubstrate cleaning apparatus for cleaning a substrate, comprising: asubstrate cleaning bath containing therein a substrate cleaning liquidcomprising at least one liquid selected from the group consisting of anaqueous solution of ammonium fluoride, and a mixture of an aqueoussolution of ammonium fluoride and hydrofluoric acid; and liquid feedingmeans for feeding a liquid comprising at least one selected from thegroup consisting of water, ammonia, aqueous ammonia, and an aqueoussolution of ammonium fluoride.
 7. The substrate cleaning apparatusaccording to claim 6, further comprising: measuring means for measuringcharacteristics of said cleaning liquid in said substrate cleaning bath;and control means for arithmetically processing a signal from saidmeasuring means to control the feeding of the liquid from said liquidfeeding means to substrate cleaning bath.
 8. The substrate cleaningapparatus according to claim 7, wherein said measuring means comprisesmeans for measuring at least one wavelength characteristic selected fromthe group consisting of an absorbance at a predetermined wavelength, aninfrared absorption spectrum, an ultraviolet absorption spectrum, and anindex of refraction.
 9. The substrate cleaning apparatus according toclaim 7, wherein said measuring means comprises means for measuring atleast one physical value selected from the group consisting of aspecific gravity and a transmittance.
 10. The substrate cleaningapparatus according to claim 7, wherein said measuring means comprisesmeans for measuring an electric conductivity.
 11. The substrate cleaningapparatus according to claim 7, wherein said measuring means comprisesat least one measurement means selected from the group consisting of amoisture titrator and liquid (ion) chromatography.
 12. The substratecleaning apparatus according to claim 7, wherein said measuring meansmeasures hydrofluoric acid concentration of said cleaning liquid. 13.The substrate cleaning apparatus according to claim 12, wherein saidmeasuring means comprises means for measuring at least one wavelengthcharacteristic selected from the group consisting of an absorbance at apredetermined wavelength, an infrared absorption spectrum, anultraviolet absorption spectrum, and an index of refraction.
 14. Thesubstrate cleaning apparatus according to claim 12, wherein saidmeasuring means comprises means for measuring at least one physicalvalue selected from the group consisting of a specific gravity and atransmittance.
 15. The substrate cleaning apparatus according to claim12, wherein said measuring means comprises means for measuring anelectric conductivity.
 16. The substrate cleaning apparatus according toclaim 12, wherein said measuring means comprises at least onemeasurement means selected from the group consisting of a moisturetitrator and liquid (ion) chromatography.